FIG. 5A is a side view showing a schematic configuration of an optical pickup device disclosed in patent document 1, and FIG. 5B is a diagram showing a light source shown in FIG. 5A, as viewed from a VB-VB line.
The optical pickup device 12 shown in FIGS. 5A and 5B is used for an optical disc device that is compatible with a first optical disc 6 and a second optical disc 6′ complying with different standards from each other, such as a DVD and a CD, and is designed to perform reading, writing, and/or erasing of information on each of the first optical disc 6 and the second optical disc 6′ with the use of light beams having wavelengths corresponding to the respective standards. Specifically, the optical pickup device 12 includes: a photo detector 9 which generates an electrical signal corresponding to the intensity of received light; a laser light source 1 which is disposed on the photo detector 9 and which emits a first laser beam having a wavelength of λ1 and a second laser beam having a wavelength of λ2 (λ2>λ1); a reflecting mirror 10 disposed on the photo detector 9; a collimating lens 4; a polarization hologram element 2; a ¼ wavelength plate 3; and an objective lens 5.
In the case of using the first optical disc 6, the laser light source 1 emits the first laser beam (wavelength λ1) from a light emitting point 1a. The emitted first laser beam is reflected by the reflecting mirror 10 fixed on the photo detector 9 toward a direction perpendicular to a detection surface 9a of the photo detector 9, and enters the collimating lens 4. The light having entered the collimating lens 4 is converted into substantially parallel light, transmitted through the polarization hologram element 2, and enters the ¼ wavelength plate 3. The ¼ wavelength plate 3 converts the entered linearly polarized light (an S-wave or a P-wave) into circularly polarized light. The light emitted from the ¼ wavelength plate 3 is converged by the objective lens 5, and forms a spot on a signal surface 6a of the first optical disc 6. The light reflected by the signal surface 6a is transmitted through the objective lens 5, and is again converted into linearly polarized light (the P-wave or the S-wave) by the ¼ wavelength plate 3, and then enters a hologram surface 2a of the polarization hologram element 2. The light having entered the polarization hologram element 2 is diffracted by the hologram surface 2a. The diffracted light is split into a first-order diffracted beam 8 and a minus first-order diffracted beam 8′ which are symmetric about a symmetry axis, i.e., an optical axis 7 of the first laser beam, and both of the diffracted beams are transmitted through the collimating lens 4 and enter the detection surface 9a of the photo detector 9.
On the other hand, in the case of using the second optical disc 6′, the laser light source 1 emits the second laser beam (wavelength λ2, λ2>λ1) from the light emitting point 1a′. The emitted second laser beam is reflected by the reflecting mirror 10 fixed on the photo detector 9, and converted by the collimating lens 4 into substantially parallel light. The light emitted from the collimating lens 4 is transmitted through the polarization hologram element 2, and is converted by the ¼ wavelength plate 3 into circularly polarized light. The circularly polarized light is converged by the objective lens 5, and forms a spot on the signal surface 6a′ of the second optical disc 6′. The light reflected by the signal surface 6a′ is transmitted through the objective lens 5, and is again converted by the ¼ wavelength plate 3 into the linearly polarized light (the P-wave or the S-wave), and the linearly polarized light enters the hologram surface 2a of the polarization hologram element 2. The light having entered the polarization hologram element 2 is diffracted by the hologram surface 2a. The diffracted light is split into a first-order diffracted beam 11 and a minus first-order diffracted beam 11′ which are symmetric about a symmetry axis, i.e., an optical axis 7′ of the second laser beam, and both of the diffracted beams are transmitted through the collimating lens 4 and enter the detection surface 9a of the photo detector 9.
FIG. 6 is a diagram showing a schematic configuration of the hologram surface 2a shown in FIG. 5A, as viewed from a VI-VI line. In FIG. 6, chain lines indicate positions, on the hologram surface 2a, where the zeroth-order diffracted beam and plus and minus first-order diffracted beams emitted from the second optical disc 6′ enter, in the case of using the second laser beam.
On the hologram surface 2a, a diffraction region of a circular shape is formed. The diffraction region is divided into four regions by two straight lines (an x-axis extending in a radial direction of the optical disc, and a y-axis extending in a direction perpendicular to the x-axis) which are perpendicular to each other at a point 20 where the optical axis 7 passes through. Further, each of the regions corresponding to respective quadrants on an x-y coordinate system is divided into three regions, and accordingly, in the quadrants, regions 21a to 21c (first quadrant), regions 22a to 22c (second quadrant), regions 23a to 23c (third quadrant), and regions 24a to 24c (fourth quadrant) are formed, respectively.
The zeroth-order diffracted beam and the plus and minus first-order diffracted beams, all of which are diffracted by the optical disc, enters the hologram surface 2a. However, the regions, where the zeroth-order diffracted beam and the plus and minus first-order diffracted beams enter, vary depending on the cases where the first laser beam is used and where the second laser beam is used.
First, in the case where the first laser beam (first optical disc 6) is used, the regions 21a, 22a, 23a, 24a, 21b, 22b, 23b, and 24b receive only the zeroth-order diffracted beam, which is reflected due to a shape of a track on the signal surface 6a and is not subjected to diffraction, whereas the remaining regions 21c, 22c, 23c, and 24d receive the zeroth-order diffracted beam and one first-order diffracted beam (either the plus first-order diffracted beam or the minus first-order diffracted beam) from the first optical disc 6.
Next, in the case where the second laser beam (second optical disc 6′) is used, the regions 21a, 22a, 23a, and 24a receive only the zeroth-order diffracted beam, which is reflected due to a shape of a track on the signal surface 6a′ and is not subjected to diffraction, whereas the remaining regions 21b, 22b, 23b, 24b, 21c, 22c, 23c, and 24c receive the zeroth-order diffracted beam and the first-order diffracted beams from the second optical disc 6′. When the objective lens 5 and the polarization hologram element 2 are displaced with respect to each other, the first-order diffracted beams enter the regions 21a, 22a, 23a, and 24a, and thus to prevent the entrance, the size of each of the regions 21b, 22b, 23b, and 24b is increased to include a predetermined amount of margin, compared to regions (chain lines) which respectively receives the zeroth-order diffracted beam and the first-order diffracted beams.
FIGS. 7 and 8 are diagrams each showing a detection pattern and a detected light distribution on the photo detector 9 shown in FIG. 5A. FIG. 7 shows a case where the first laser beam is used, whereas FIG. 8 shows a case where the second laser beam is used.
For convenience of explanation, positions on the detection surface 9a are indicated with the use of an x-y coordinate system. That is, as shown in FIGS. 7 and 8, an intersection point between an optical axis 7 of the first laser beam and the detection surface 9a is defined as a point 90, and two straight lines perpendicular to each other at the intersection point are defined as an x-axis and a y-axis. Further, point 90′ is an intersection point between the optical axis 7′ of the second laser beam and the detection surface 9a. 
In a region on the y-axis positive side on the detection surface 9a, focus detection cells F1a, F2a, F1b, F2b, F1c, F2c, F1d, and F2d are located. Polarities of the focus detection cells are different from one another. Further, in the vicinity of and on the y-axis negative side from the above focus detection cells, tracking detection cells S1b, S1c, S1d, S1e, S2b, S2c, S2d, and S2e, each having a square shape, are located. The tracking detection cells are used in the case where the first optical disc is used. Further, on the x-axis positive and negative sides from the focus detection cells, off-track compensation detection cells S1a and S2a are located. The off-track compensation detection cells are used in the case where the second optical disc is used. The focus detection cells, the tracking detection cells, and the off-track compensation detection cells are located so as to be symmetric about the y-axis, respectively.
Further, a region on the y-axis negative side on the detection surface 9a, tracking detection cells 3T1, 3T2, 3T3, and 3T4, each having a square shape, are located so as to be symmetric about the y-axis.
Hereinafter, relation between light entering the respective regions on the hologram surface 2a shown in FIG. 6 and light spots on the detection surface 9a will be described.
In the case where the first laser beam is used, the laser beam, which is from the optical disc 6a and is entering each of the quadrants on the hologram surface 2a, is diffracted and converged as follows (see FIG. 7).
<First Quadrant (First Laser Beam)>
The first-order diffracted beam, which is diffracted by the regions 21a, 21b, and 21c in the first quadrant, forms beam spots 31aB, 31bB, and 31cB on the detection cells S1b, S2e, and S1c, respectively. Further, the minus first-order diffracted beam, which is diffracted by the regions 21a, 21b, and 21c, forms beam spots 31aF, 31bF, and 31cF, respectively, on the detection cell 3T1.
<Second Quadrant (First Laser Beam)>
The first-order diffracted beam, which is diffracted by the regions 22a, 22b, and 22c on the second quadrant, forms beam spots 32aB, 32bB, and 32cB, on the detection cell S2b, S1e, and S2c, respectively. Further, the minus first-order diffracted beam, which is diffracted by the regions 22a, 22b, and 22c, forms beam spots 32aF, 32bF, and 32cF, respectively, on the detection cell 3T2.
<Third Quadrant (First Laser Beam)>
The first-order diffracted beam, which is diffracted by the third quadrant regions 23a, forms a beam spot 33aB on a position outside the detection cells, and the first-order diffracted beam, which is diffracted by the regions 23b and 23c, forms beam spots 32bB and 33bB on a boundary between the detection cells F2a and F1b. Further, the minus first-order diffracted beam, which is diffracted by the regions 23a, 23b, and 23c, forms beam spots 33aF, 33bF, and 33cF, respectively, on the detection cell 3T3.
<Fourth Quadrant (First Laser Beam)>
The first-order diffracted beam, which is diffracted by the regions 24a on the fourth quadrant, forms a beam spot 34aB on a position outside the detection cells, and the first-order diffracted beam, which is diffracted by the regions 24b and 24c, forms beam spots 34bB and 34cB on a boundary between the detection cells F2c and F1d. Further, the minus first-order diffracted beam, which is diffracted by the regions 24a, 24b, and 24c, forms beam spots 34aF, 34bF, and 34cF, respectively, on the detection cell 3T4.
Next, in the case where the second laser beam is used, the light, which is from the optical disc 6a′ and is entering the respective quadrants on the hologram surface 2a, is diffracted and converged as follows (see FIG. 8).
<First Quadrant (Second Laser Beam)>
The first-order diffracted beam, which is diffracted by the regions 21a on the first quadrant, forms a beam spot 41aB on the detection cell S1a, and the first-order diffracted beam, which is diffracted by the regions 21b and 21c, forms beam spots 41bB and 41cB on positions outside the detection cells. The minus first-order diffracted beam, which is diffracted by the regions 21a, 21b, and 21c, forms beam spots 41aF, 41bF, and 41cF, respectively, on the detection cell 3T1.
<Second Quadrant (Second Laser Beam)>
The first-order diffracted beam, which is diffracted by the region 22a on the second quadrant, forms a beam spot 42aB on the detection cell S2a, and the first-order diffracted beam, which is diffracted by the regions 22b and 22c, forms beam spots 42bB and 42cB on positions outside the detection cells. Further, the minus first-order diffracted beam, which is diffracted by the regions 22a, 22b, and 22c, forms beam spots 42aF, 42bF, and 42cF, respectively, on the detection cell 3T2.
<Third Quadrant (Second Laser Beam)>
The first-order diffracted beam, which is diffracted by the region 23a on the third quadrant, forms a beam spot 43aB on the detection cell S2a, and the first-order diffracted beam, which is diffracted by the regions 23b and 23c, forms beam spots 43bB and 43cB on a boundary between the detection cell F2a and F1b. Further, the minus first-order diffracted beam, which is diffracted by the regions 23a, 23b, and 23c, forms beam spots 43aF, 43bF, and 43cF, respectively, on the detection cell 3T3.
<Fourth Quadrant (Second Laser Beam)>
The first-order diffracted beam, which is diffracted by the region 24a on the fourth quadrant, forms a beam spot 44aB on the detection cell S1a, and the first-order diffracted beam, which is diffracted by the regions 24b and 24c, forms beam spots 44bB and 44cB on a boundary between the detection cells F2c and F1d. Further, the minus first-order diffracted beam, which is diffracted by the regions 24a, 24b, and 24c, forms beam spots 44aF, 44bF, and 44cF, respectively, on the detection cell 3T4.
Next, a focus error detection method and a tracking error detection method will be described.
A diffraction pattern on the hologram surface 2a is formed such that a convergence point of the first-order diffracted beam is positioned inside a substrate of the photo detector 9, and that a convergence point of the minus first-order diffracted beam is positioned at a position on the optical disc side from the detection surface 9a. When the objective lens is shifted toward a direction parallel to the optical axis to change a working distance of the objective lens, the convergence point of each of the first-order diffracted beam and the minus first-order diffracted beam is also shifted toward a direction parallel to the optical axis, and thus, the size of each beam spot formed on the detection surface 9a changes. Therefore, in accordance with the size of each beam spot formed on the detection surface 9a, it is possible to detect a focus signal (so called a spot-size method).
Some of the above-described detection cells are electrically connected to one another to obtain the following eight signals.
F1=signal obtained in detection cell F1a+signal obtained in detection cell F1b+signal obtained in detection cell F1c+signal obtained in detection cell F1d 
F2=signal obtained in detection cell F2a+signal obtained in detection cell F2b+signal obtained in the detection cell F2c+signal obtained in the detection cell F2d 
T1=signal obtained in detection cell 3T1
T2=signal obtained in detection cell 3T2
T3=signal obtained in detection cell 3T3
T4=signal obtained in detection cell 3T4
S1=signal obtained in detection cell S1a+signal obtained in detection cell S1b+signal obtained in detection cell S1c+signal obtained in detection cell S1d+signal obtained in detection cell S1e 
S2=signal obtained in detection cell S2a+signal obtained in detection cell S2b+signal obtained in detection cell S2c+signal obtained in detection cell S2d+signal obtained in detection cell S2e 
Calculation means (not shown) provided on the photo detector 9 performs calculations of the following formulas (1) to (4) with the use of each of the detected signals F1, F2, T1, T2, T3, T4, S1, and S2 so as to obtain a focus error signal FE, a signal TE1, a signal TE2, and a reproduction signal RF on the optical disc signal surface.FE=F1−F2  (1)TE1=S1−S2  (2)TE2=(T2+T3)−(T1+T4)  (3)RF=T1+T2+T3+T4  (4)
Further, a tracking error signal TE for tracking control is obtained in accordance with the following formulas (5) and (6).TE=TE1  (5)
(In the Case of Using the First Laser Beam)TE=TE2−α×TE1  (6)
(In the case of using the second laser beam, wherein α is a constant)
FIGS. 9A and 9B are schematic diagrams showing changes in offset amounts of signals TE1 and TE2 in the case where the objective lens 5 and a polarization hologram substrate 2 are decentered in the radial direction of the optical disc 6′ and in the case where the second laser beam is used. FIG. 9A indicates the signal TE2 represented by the above formula (3), and FIG. 9B indicates the signal TE1 represented by the above formula (2). The horizontal axis indicates an amount of decentering of each of the objective lens 5 and the polarization hologram substrate 2 in the radial direction, where the optical axis is set as the reference. The vertical axis indicates an offset amount included in each of the signals.
Generally, light intensity is strong on and in the vicinity of the optical axis, and decreases in a portion of the light which is increasingly distant from the optical axis. That is, since the light intensity distributes unevenly, the signals TE2 and TE1 are offset due to decentering of the objective lens 5 (i.e., decentering relative to the light intensity distribution). Therefore, in the case where the second laser beam is used, the offset included in the signal TE2 needs to be electrically compensated with the use of the signal TE1. Specifically, in accordance with formula (6), the signal TE1 amplified by an appropriate weight (constant α) is subtracted from the signal TE2, whereby the offset of the tracking error signal TE, which is caused by the decentering of the objective lens 5 and the polarization hologram substrate 2, can be cancelled. The tracking error signal TE is generated in this manner, whereby it is possible to prevent off-track from being caused by a change in the offset amount at the time of tracking control.
The above-described uneven light intensity distribution also occurs in a similar manner in the case where the first laser beam is used. More specifically, due to decentering of the objective lens 5 and the polarization hologram substrate 2, the light distribution on the regions 21a, 21b, and 21c (the first quadrant on the hologram surface 2a) and the light distribution on the regions 22a, 22b, and 22c (the second quadrant on the hologram surface 2a) become asymmetric. As indicated by the above formula (5) (formula (2)) and FIG. 7, in the case where the first laser beam is used, the tracking error signal TE is generated based on the intensity of the first-order diffracted beam diffracted by the first quadrant and the second quadrant on the hologram surface 2a, and thus the asymmetry between the light distribution in the first quadrant and that in the second quadrant leads to deterioration in the tracking error signal TE.
In the optical pickup device 12 according to this example, in the case of using the first laser beam, electrical connection between tracking detection cells are made elaborately, instead of using an offset compensation signal, whereby the offset is compensated. Specifically, the spot 31bB generated by the region 21b in the first quadrant and the spot 32bB generated by the region 22b on the second quadrant are interchanged with each other, and a difference signal is generated. As a result, asymmetry of the light intensity distribution is cancelled in accordance with a calculation based on formula (2), and offset of the tracking error signal TE is reduced.
The optical pickup device 12 as above described is compatible with optical discs of two standards, and is capable of performing tracking control without causing off-track even in the case where the objective lens 5 and the polarization hologram substrate 2 are decentered in the radial direction of a disc.    [Patent document 1] International Publication 2007/072683 Pamphlet